1. Field of the Invention
The present invention relates to data bus networks. More particularly, this invention is a method and apparatus for implementing a redundant data bus network utilizing standard components. The present invention is particularly well-suited for use in a data network interconnecting avionics components onboard an aircraft.
2. Description of the Prior Art
Networking architectures connecting computers are well known in the prior art. Microprocessor based systems are widely interconnected by data networks, allowing for high speed data sharing, parallel processing and communication.
A number of methods and devices that allow computers to communicate exist. Ethernet, for example, is described in IEEE standard 802.3 and in U.S. Pat. No. 4,063,220 issued to Metcalfe et al. on Dec. 13, 1977, which is hereby incorporated by reference. Because ethernet is the world's most popular local area networking scheme, a number of low-cost components for implementing ethernets are widely available throughout the world.
The ethernet standard encompasses several varieties of cabling. 10base2 ethernet, for example, uses a coaxial cable of not more than 185 meters in length that is capable of transmitting ten megabits of information per second. 10baseT ethernet is also capable of transmitting ten megabits of information per second, but utilizes shielded twisted pair cables. All forms of ethernet utilize passive cables; devices communicating via the ethernet must contain active components. Typically, computing systems communicate over the ethernet through a standard network interface controller (NIC) that is well-known in the art. NICs are active devices that normally receive power from their associated computing hosts.
FIG. 1 describes a prior art NIC 106 containing transceiver 101 and a backplane interface 104. The transceiver 101 is capable of transmitting and receiving analog signals via an ethernet 107 and of converting these analog signals to digital equivalents. Backplane interface 104 is capable of transferring data between the NIC and the backplane bus 105, which is a component of a processing device such as a personal computer. Transceiver 101 and backplane interface 104 are coupled by a pair of conductors 119L and 119R, one for data to be transmitted on the ethernet and one for data received from the ethernet. Typically, communications between the backplane and the ethernet are controlled by a well-known system network interface controller (SNIC) 103 that transfers data between interface 104 and transceiver 101. An isolation circuit 102 is typically provided between transceiver 101 and SNIC 103 to insure signal quality by eliminating ground loops and faults. The isolation means 102 also serves to disable communications by the NIC 106 when a malfunction is detected.
Ethernet is an asynchronous protocol using a Carrier Sense Multiple Access with Collision Detect (CSMA/CD) access scheme. No central host controls access to the network, and no clocking scheme is utilized to control access to the conductor. Rather, NICs with data to transmit first check the ethernet to determine if it is busy transferring data from another host. If the ethernet is free, the NIC will transmit the data immediately. If the ethernet is busy, however, the NIC will wait a random period of time before re-checking for ethernet traffic. The ethernet standard limits the duration and frequency of data transmissions. If two NICs on the same ethernet begin transmitting simultaneously, each will sense that data has "collided" and will re-transmit after a random period of time.
When the ethernet remains very busy for a prolonged period of time, collisions and re-transmissions become more frequent. Each re-transmission creates additional traffic on the ethernet, and collision frequency can increase exponentially. As collision frequency increases, the time necessary to transmit data across the bus also increases. Because the CSMA/CD access scheme is asynchronous and non-deterministic, prior art ethernets are unsuitable for use in critical applications where immediate data transmissions are essential. In avionics applications, for example, a windshear detector or collision avoidance system may need to transmit an immediate warning to an autopilot system or to an output device. Because such signals may affect the safety of the aircraft, it is essential that they be transmitted without delay. Even a potential risk of transmission delay is unacceptable in such applications. Moreover, avionics components require certain information to be provided synchronously. Attitude and altitude readings, for example, must be provided to avionics components at regular intervals without fail. Inexpensive ethernet networks provide high bandwidth and proven physical layer characteristics; however, ethernet's asynchronous nature is too unstable for environments where data integrity and reliability are critical.
Modern aircraft include a number of digital avionics components such as traffic alert and collision avoidance systems (TCAS), autopilots, flight management systems (FMS) and integrated radio systems communicating over a system bus. Because the avionics system bus is essential to the intercommunication of avionics components and therefore the safety of the aircraft, the system bus must be highly reliable and fault tolerant.
Prior art avionics buses have utilized redundant conductors to improve reliability. Network standards such as the Avionics System Communications Bus (ASCB) allow avionics components within an aircraft to work together safely and efficiently. ASCB is a synchronous networking protocol, meaning that each component has an allotted share of guaranteed bandwidth. Referring to FIG. 2, ASCB includes four conductors 107 connecting two sets of avionics components 110 corresponding to a pilot's side and a copilot's side. Each avionics component 110 transmits data on an onside data bus and receives data via both the onside and cross-side data buses. In FIG. 2, conductor 107L is the onside bus and conductor 107R is the cross-side bus for components 110A, 110B, 110C and 110D. For components 110E, 110F, 110G and 110H, conductor 107R is the onside bus and conductor 107L is the cross-side bus. Thus the onside bus for components on one side of the aircraft is the cross-side bus for components on the other side of the aircraft.
Two backup buses 107LB and 107LR provide added redundancy by connecting those components on the same side of the aircraft. Each avionics component 110 is therefore in communication with at least three conductors: components send and receive data via the on-side and backup buses, and receive data from components on the opposite side of the aircraft via the cross-side bus.
While ASCB and other avionics buses such as MIL-STD-1553B provide the reliability necessary for avionics applications, these bus architectures have a number of marked disadvantages. Most notably, prior art avionics buses provide significantly lower bandwidth than comparable non-avionics bus technologies. Moreover, prior art buses are relatively expensive to implement because they have not been readily adopted for non-avionics applications. The specialized nature of prior art avionics buses has resulted in high costs of design, manufacturing and support. Moreover, the specialized nature of prior art avionics buses makes re-configuration difficult. System changes, expansion, and upgrades are complicated, expensive and time-consuming.